As technology has progressed, a major challenge to computer manufacturers has been to increase the amount of information that can be stored in a computer magnetic disc drive. Because data recorded in tracks of a magnetic disc drive must be followed with extreme precision, a disc drive actuation system must be very accurate. FIG. 1A shows a top view of prior art disc drive actuation system 1. Information is stored on concentric tracks 3 of magnetic recording disc 4. Voice coil motor 6 is arranged to rotate actuation arm 8 about pivot axis 10 so as to place read/write head 14 (FIG. 1B) over a track 3. Flexure 2 is suspended from suspended load beam 16. Read/write head 14 is embedded in slider 12 and rides above the surface of disc 4 on an air cushion and is pressed towards disc 4 by suspended load beam 16. Slider 12 possesses aerodynamic features to stabilize its motion on the air cushion.
Increasing track density and decreased access times have increased the demands on voice coil motor 6 to accurately position read/write head 14 over track 3. In addition, the need for faster access times makes it necessary to operate the actuation system at frequencies approaching resonance in the actuation arm. Increased track density, vibrations in the actuation arm, and vibrations in the suspension of the rotating magnetic disc can cause unacceptable tracking errors. Voice coil actuation is near the limit of the technology.
A read/write head is tracking accurately when it is positioned above the center of a track. Methods have been devised to improve tracking accuracy by inserting a piezoelectric actuator to act in conjunction with the voice coil motor to position the read/write head above the track. U.S. Pat. No. 6,052,251 (Mohajerany), U.S. Pat. No. 6,046,888 (Krinke) and U.S. Pat. No. 6,025,975 (Fard) disclose piezoelectric actuators at various positions along the actuation arm and suspended load beam, but back from the read/write head. Each of these techniques may result in improved tracking accuracy. However, because the piezoelectric actuators are located at some distance away from the read/write head, vibrations in the actuator arm are added to the piezoelectric actuator output causing read/write head motion that cannot be adequately controlled. Additionally, high frequency corrections that may be necessary to eliminate the tracking errors cause by bearing vibrations of the rotating disc are not effective when these frequencies approach the resonant frequency of the arm.
Boutaghou, U.S. Pat. No. 5,898,541, discloses a piezoelectric actuator that is positioned near the read/write head. By reference to FIG. 2 of U.S. Pat. No. 5,898,541, the piezoelectric microactuator is placed between the slider and flexure tab surface. By placing the microactuator closer to the read/write head, many of the vibration and tracking problems associated with the devices disclosed in the Mohajerany, Krinke and Fard devices are greatly reduced. However, in the Boutaghou device, there is undesirable friction between the load beam and the slider that impedes the movement of the slider relative to the load beam when the microactuator moves the slider relative to the load beam. If Boutaghou's optional compliant shear layer is utilized it will also impede the smooth motion of the slider by applying a torque to the slider as the slider rotates about the yaw axis.
Kimura, U.S. Pat. No. 4,583,135, issued in 1986 describes a tracking device in which a magnetic head is controlled with oppositely expanding and contracting piezoelectric elements sandwiched between the magnetic head and two legs of a rigid frame driven by a step motor.
What is needed is a better actuation system for improved tracking control of a magnetic disc drive read/write head.